This invention relates to the field of heat exchangers, and more particularly to heat exchangers which are uniquely suited to be used with solar energy collector systems.
A widely known and currently popular application of solar energy collector systems is to provide primary or auxiliary heating or cooling for residential uses. In such systems, a liquid heat transfer medium is typically circulated through the solar energy collector, both to absorb solar energy for heating purposes during the day, and to release energy at night for nocturnal cooling purposes. In order to provide space heating, the stored heat in the liquid heat transfer medium is circulated through a system of coils and the heat is thereafter transferred by a fan. Providing solar heat for a residential hot water tank is accomplished by circulating cold water from the bottom of a hot water tank through a pipe which is in heat exchange relationship with a pipe from the solar collector. For safety reasons, hot water heating for domestic purposes requires heat exchangers having a double wall construction so that a leak in either one of the domestic hot water pipes or the pipe containing water from the solar collector will not result in a contaminated domestic water supply.
Many homes also have a swimming pool or spa which requires continuous circulation and filtration in a flow path which is separate from the domestic hot water tank. Since spas and swimming pools have much larger volumes than domestic hot water tanks, flow rate requirements differ widely. For these reasons, residential solar energy systems providing heat energy for both potable hot water tanks and spas or swimming pools have previously required separate heat exchangers. One object of the present invention is to provide a single heat exchanger unit having sufficient effectiveness to be adapted for use with solar energy systems and which permits heat exchange with a plurality of sources.
As earlier mentioned, when a potable water supply such as domestic hot water is to be heated by non-potable collector fluid, double walled heat exchangers are often required. As well known in the art, however, the resistance to heat transfer is higher in a double walled heat exchanger as opposed to single walled heat exchanger due to the added material thickness. Thus, the heat exchanger's effectiveness is decreased when the double wall design is employed. This resistance to heat transfer may be minimized by using highly conductive materials such as copper to form the double walls, and also by increasing the area of surface contact between the walls which are in heat exchange relationship. Nevertheless, double walled heat exchangers are usually larger than single walled heat exchangers having similar heat exchange effectiveness, and the double walled construction is considerably more complex.
One design which is typical of counter-flow heat exchangers having double walled construction is disclosed in U.S. Pat. No. 4,067,314 to Bollefer. The heat exchanger disclosed therein employs two helically wound coils superposed in heat exchange relationship within a tank. This device is different from the present invention in several respects. First, the Bollefer device does not disclose the four concentric helixes of the present invention, which offer higher heat exchanger effectiveness for a given sized heat exchanger. Also, the heat exchanger of the present invention employs a third heat transfer medium within a space enclosed by the tank and surrounding the coils. Further, and very importantly, the Bollefer device is not capable of providing energy to a second heating need or demand through a third heat transfer fluid.
It would therefore be an improvement to provide a high efficiency heat exchanger adapted for use with solar energy in a double wall heat exchange relationship with an energy load such as a potable water tank. It would be a further improvement to provide a heat exchanger adapted for use with solar energy systems and which is capable of exchanging heat energy with a plurality of energy loads via separate flow paths.
Other devices which disclose heat exchangers which may have some general relevance to the present invention are as follows:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 1,425,174 Cartter et al. 2,149,737 Jewell 2,339,229 Wyllie 2,653,014 Sniader 3,131,553 Ross 4,172,491 Rice 4,257,479 Newton ______________________________________
U.S. Pat. No. 1,425,174 to Cartter et al. discloses a solar heat collecting apparatus which includes a single walled heat exchanger. The heat exchanger includes a plurality of coils extending vertically to different heights within a heat storage tank. The coils are in flow communication at their lower ends and deliver heated water at different heights to a water tank.
U.S. Pat. No. 2,149,737 to Jewell discloses a heat exchanger of the shell and tube type comprising a shell having a plurality of coils arranged in two sections. Each section comprises a first coil of large size and a second coil of smaller size nested within the first coil.
U.S. Pat. No. 2,339,229 to Wyllie discloses an apparatus for cooling potable liquids on draft including beer and other carbonated beverages and water. A tank containing liquid to be cooled is surrounded by a coil containing a refrigerent which is in turn surrounded in heat conducting contact by a water cooling coil. A discharge conduit connected to the outlet of the tank is in heat conducting contact with at least a portion of the refrigerant coil, but is out of contact with the water cooling coil.
U.S. Pat. No. 2,653,014 to Sniader discloses a liquid cooling and dispensing device wherein a plurality of coils are spirally wound about a reservoir tank in superposed relationship so that leaks in the tank or the coils may be quickly detected.
U.S. Pat. No. 3,131,553 to Ross discloses a refrigeration system including a combination condenser and heat exchanger having a pair of helically wound coils and a casing defining an inner heat exchange chamber and an outer condenser chamber.
U.S. Pat. No. 4,172,491 to Rice discloses a method for operating a heat storage-heat exchange system. Heated fluid is passed through a bed of heat storage medium, creating a hotter portion of the medium. A fluid to be heated is passed through the medium counter-current to the heating fluid to create a cooler portion of the medium. A fluid is also passed from within the system through an intermediate portion of the medium to steepen the temperature gradient between hotter and cooler portions.
U.S. Pat. No. 4,257,479 To Newton discloses a heat exchanger for a solar energy collector wherein the exchanger has an open top inner tank enclosed in a closed outer tank. A coil connected to the hot water tank is disposed within the annular space between the inner and outer tanks. None of the above references, however, discloses a device which accomplishes the object of the present invention.